The first one, .150, is the build that comes with the new Lumia 1520/1320. The newest one is just bug fixes, nothing new added to it. In my opinion I think the next update we get is a beta of 8.1 in February or March with build now being in April. Or build could be when we get a beta/preview for 8.1 and we see a small update again to get ready for it.

This all depends on how update 3 goes out officially through the carriers first and if they want to wait for update 3 to finish going out before they seed a new preview build.